All communications networks have followed one of two broad development paths. The first path is represented by the work of the telephone industry and the second path is represented by the work of the computer industry. The terminology used to describe the communications networks and the switching theory associated therewith may be slightly different when working within one of the two domains. For purposes of clarity, the description of the present invention and the description of the background of the invention will use the language of telephony.
In telephone terms, there are three basic elements of a communications network: terminals, transmission media, and switches. Terminals are the input and output equipment which attach users of the network to the network equipment. Transmission media include wire cables, coaxial cables, fiber optic cables, microwaves, and a variety of other media. The switch is the device that makes the network function by providing connections between the various terminals by way of the transmission media.
Within the realm of switching as a technique for interconnect there are two broad classifications of method: time division switching and space division switching. Time division switching involves the sharing of a single transmission medium by multiple users who have access to the medium during different periods of time. Time division multiplexers provide a sequence of time slots during which individual sources can transmit and during which individual destinations can receive. Time division switching is accomplished by selecting appropriate pairings of input time slots to output time slots so as to connect willing correspondents.
Space division switching is based on the sharing of multiple transmission paths. A space-division switch provides a private transmission path between an input and an output that is set up on request, used exclusively by the correspondents, and then released. Space division switching allows many different forms of communication to be carried on over the transmission path, since space division switching is relatively indifferent to the kind of use that is being made of a connection. In contrast to this, time division switching requires clock synchronization, common data formats, and common transmission speeds on the shared link.
The first space division switches were implemented in the telephone industry as crossbar switches. A typical crossbar switch is a crosspoint array of M.times.N conductors where the intersections of conductors formed crosspoint switches. This switching matrix can be used to connect any one of the quantity (M) inlets to any one of the quantity (N) outlets. An inlet is connected directly to an outlet through a single crosspoint connection. The crosspoints of the crossbar switch were originally mechanical contacts that were operated by magnets in order to set up and hold the connection. Today it is common to find the crosspoints implemented with computer logic elements taking the place of mechanical contacts and magnets.
One of the disadvantages of the crossbar switch is that an excessive number of crosspoints (M.times.N) are required to implement a large switch. The largest switches which are typically used in the telephone industry are on the order of 300 crosspoints in a space division switch. Designers have learned, however, that one way to improve the effectiveness of crossbar switching is to combine crossbar assemblies into multi-stage designs. This strategy was developed in 1952 by Charles Clos of Bell Laboratories for sizing the various crossbar assemblies to effect a most effecient usage of crossbars in large designs and is well known to those skilled in the art.
Even using multi-stage designs for a space domain switch, the number of crosspoints needed to implement large designs continued to be excessively large. Eliminating the number of crosspoints caused other problems such as reducing the traffic-carrying capabilities of the system and increasing the effect of component failures on comunications through the system. Limiting the number of crosspoints in a multi-stage design also caused blocking problems when the number of users increased beyond a limited capacity of the multi-stage switch.